An in vitro approach for studying organ and species differences in the activation of chemical carcinogens has been previously developed. Both intact cells and cell homogenates have been used for metabolic activation although intact primary cells are most frequently used as they best simulate in vivo metabolism. To assess biological activity, the multiple genetic endpoints of toxicity, mutation and SCE induction in V79 cells and reversion of S. typhimurium are used. In addition, HPLC analysis of metabolites formed by primary cells from different organs and species have been conducted. Most of the work during the past year have focused on differences in liver and bladder cell activation/metabolism from rat, dog, and bovine. One outcome of this research has been the observed species difference in the relative capability of these organs to activate carcinogenic aromatic amines. Results from another study have indicated the differences between rat and hampster liver cells in the activation of nitrosamines and aromatic amines and the relative sensitivities of the genetic endpoints listed above to the activated intermediates.